This invention relates, in general, to semiconductor devices, and more particularly, to transistors having high breakdown voltage and low "on" resistance.
The design and fabrication of semiconductor devices involves the balancing of various electrical parameters to provide a device that meets the requirements of a particular customer. With power transistors, two of the most important electrical parameters are breakdown voltage and "on" resistance. Breakdown voltage (BV) generally refers to the maximum voltage that can be placed between the source and drain of a transistor while the transistor is in an "off" or non-conducting state (i.e., the voltage on the gate is less than the threshold voltage of the device). "On" resistance, also referred to as R.sub.ds on, is the resistance measurement between the drain and source terminals of a transistor when the transistor is in a conductive state (i.e., the voltage on the gate is equal to or greater than the threshold voltage of the device).
Historically, to increase the breakdown voltage of a lateral power transistor, the amount of dopant between the source and drain regions near the surface of the substrate is reduced. However, the reduction in dopant concentration results in an increase in the "on" resistance (R.sub.ds on) between the source and drain terminals. Conversely, with everything else being equal, improvements in R.sub.ds on typically result in a device that has a lower breakdown voltage.
Accordingly, a need exists to provide a semiconductor device that has either improved breakdown voltage or R.sub.ds on without compromising the other parameter.